Guarding the entrance to the bay, the circular splat of Mamula is only 200m in diameter. It’s named after the Austro-Hungarian general who in the mid-19th century created the fort still standing on the island, which became an infamous Italian prison during WWII.
The island’s a stop on one of the Kayak Montenegro day tours as well as many of the boat tours of the bay. Otherwise it’s easily reached by taxi boat, or you can hire a kayak and paddle there in about one hour.